Note. Colored lines indicate individual participants. Black lines indicate the predicted interaction effect based on the mixed effect model depicted in Table 4. The waitlist period occurred between sessions W1 and T1, while the heart rate variability biofeedback intervention period took place between T1 and T5. BDI – Beck’s Depression Inventory II; *** – p<.001; ns – not significant.
3.4 Stress and Anxiety
The best model predicting DASS values included the RMSSD and the DASS SCALE (anxiety, stress or depression) on top of TREATMENT, TIME and their interaction (see Table 5). There was no three-way interaction of TREATMENT * TIME * SCALE. The model was:
Value ~ treatment*time + scale + RMSSD + (1|vpn) + (1|item).
Post-hoc Tukey testing of the TREATMENT * TIME interactions showed a significant improvement in the intervention period, d = -0.19, tratio(1573) = -3.88, p < .001, but not in the waitlist period, d = 0.09, tratio(1567) = 1.37, p = .17 (see Figure 5).
Figure 5. Course of Anxiety/Stress symptoms